The present invention was initially developed as an improvement upon the blown bubble process for forming thin heat sealable oriented films of improved physical characteristics. However, upon review those skilled in the art will appreciate numerous other applications.
The blown bubble process has been utilized for many years and is well known by those skilled in the art. This process is disclosed in numerous patents, exemplary of which is U.S. Pat. No. 3,022,543 to W. G. Baird, Jr. et al, and is hereby incorporated by reference.
Generally speaking, the blown bubble process utilizes the application of inner pressure to stretch and thus orient a heat sealable polyolefin material which has been melt extruded in tubular form, cooled and reheated to its orientation temperature. The inner pressure is applied by the trapping of a gaseous bubble inside the extruded tubular structure so as to stretch the heated polyolefin material to orient the molecular infra-structure of the polymer. Reheating of the extruded material may be accomplished by means well known in the art such as a hot air oven, hot water or hot oil bath. Of course, the reheating means may vary depending upon the orientation temperature of the extruded material.
Many of the heat sealable polyolefin materials and, in particular homopolymers and copolymers and blends thereof of monoalpha polyolefins having from 2-8 carbon atoms, which have heretofore been utilized in the blown bubble process weld when heated to the temperature range in which the material can be oriented. Welding as used herein refers to the tendency of two or more surfaces of a given material to adhere to one another at certain temperatures and pressures. Accordingly, a problem which has arisen in past applications of the blown bubble process with extruded tubular materials which weld upon heating to their orientation temperature range is the resistance of the materials to separation by internal pressure as applied by the internally trapped gaseous bubble.
It is known in the art that the utilization of a film forming air mandrel, as opposed to a mandrel where the extruded film comes into actual contact with the forming mandrel, for supporting and forming the cooling melt extruded tubular material shortly after extrusion provides an increase in the optical properties of the extruded material. It is believed that this increase in clarity results because the inner surface of the cooling tubular material extruded over the air mandrel never contacts the mandrel and, therefore, is not submitted to the minute scratching, etc. which adversely affects a film's overall optical properties.
A major drawback of the blown film process when utilized in conjunction with materials which weld in their orientation temperature range is the limitation on film thickness. Heretofore, a film thickness of less than 0.50 mils was not consistently obtainable by utilization of the blown bubble process when applied to tubular extruded structures having as an inner surface a weldable polyolefin material. The limitation on thickness directly resulted from the internal welding of the surface of the heated tubular materials because the strength of the internal structure of the thin film material having a thickness of less than 0.50 mils was closely approximated or exceeded by the strength of the weld. Thus, in the past, the structural integrity of thin tubular films having an inner weldable surface and a thickness of less than 0.50 mils was compromised upon inflation of the tubular structure and subsequent attempted separation of the welded layers.
Unfortunately, it has been found that utilization of an air mandrel to increase film clarity as discussed above further limits the lowest obtainable film thickness since films processed utilizing the air mandrel exhibit increased welding. It is believed that this increased welding arises from improved inner surface contact of the collapsed tubular film which results from the more uniform, scratchless, film surface created through processing by the air mandrel. Therefore, utilization of an air mandrel apparently conflicted with the goal of obtaining a thinner film.
Yet another problem which confronted those skilled in the art in attempting to obtain a thin film of increased optical properties and decreased thickness should be noted. The problem was that the heat sealing characteristics of the thin film could not be compromised. This fact apparently ruled out the utilization of materials, apparatus or processes which were known to adversely affect the heat sealing characteristics of the thin film.
Lastly, it was also desired that the thin film would be able to be easily inked or printed upon. Thus, utilization of materials, apparatus or processes which were known to adversely affect the ink adhesion of the thin film appeared to be negated to those skilled in the art.
In summary, it can be stated that while those in the art were questing for thin films having increased clarity, decreased thickness and adequate heat sealing and inking characteristics, an impass appeared to have formed in that the air mandrel which could provide increased clarity unfortunately increased welding and, consequently, the minimum film thickness.
Accordingly, it is an object of the present invention to provide an improvement to the blown bubble process for orienting thin films by eliminating or greatly reducing internal welding during the process.
Another object of the present invention is to provide a thin polyolefin film having a thickness of less than 0.50 mils.
A still further object of the present invention is to provide a polyolefin film having improved clarity.
Yet another object of the present invention is to provide a polyolefin film having good heat seal characteristics.
An additional object of the present invention is to provide a polyolefin film having good ink adhesion.
A further object of the present invention is to provide a means for eliminating internal welding during the processing of thin tubular materials.
Still further objects and the broad scope of applicability of the present invention will become apparent to those of ordinary skill in the art from the details given hereinafter. However, it should be understood that the detailed description and specific examples which indicate the presently preferred embodiments of the invention are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those of ordinary skill in the art from this detailed description.